The present invention relates to a semiconductor memory device having a plurality of dynamic memory cells each composed of one switching transistor and one storage capacitor coupled to the transistor, and more particularly, to the capacitor structure thereof.
To enhance the capacitance of the storage capacitor with a high integration density, various types of the memory devices have been proposed. One type of device features a MOS type capacitor provided over the gate electrode of the transistor and connected to one of the source and drain regions of the transistor, and a digit line extends over the MOS capacitor and connected to the other of source and drain regions through a contact hole formed in an insulating layer. The structure, however, necessitates an adequateinterval between the contact hole and the gate electrode to prevent a short circuit between the gate electrode and the digit line within the contact hole, and it hinders the miniaturization of the device. To cope with the problem, a new memory device has been proposed by Masao Taguchi et al. in U.S. Pat. No. 4,419,743, in which a digit line is coupled to the upper electrode of the MOS type capacitor which is connected to one of source and drain regions of the transistor, and the other of source and drain regions is coupled to a reference potential line such as the V.sub.DD line or the ground line, as shown in its FIGS. 4 and 8A. The digit line is not connected to the source or drain region of the transistor, and therefore, the contact hole mentioned above is not necessary. The MOS type capacitor of the newly proposed memory device, however, depends mainly on an area over the transistor forming portion, that is, over the gate electrode of the transistor. Therefore, a sufficiently large capacitance of the MOS type capacitor cannot be obtained. Further, the MOS type capacitor is formed on an uneven surface of the insulating layer, obstructing reliable patterning of the upper electrode of the MOS type capacitor and of the digit line coupled to the upper electrode of the capacitor.